This invention relates generally to the development of electrostatic images, and more particularly concerns a scavengeless development system in which an asymmetrical electrical bias is applied on the donor development roller.
The invention can be used in the art of color electrophotographic printing. Generally, the process of electrophotographic printing includes sensitizing a photoconductive surface by charging it to a substantially uniform potential. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to a desired image. The selective dissipation of the charge leaves a latent charge pattern that is developed by bringing a developer material into contact therewith. This process forms a toner powder image on the photoconductive surface which is subsequently transferred to a copy sheet. Finally, the powder image is heated to permanently affix it to the copy sheet in image configuration. Two component and single component developer materials are commonly used. A typical two component developer material comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles having an electrostatic charge so that they will be attracted to, and adhere to, the latent image on the photoconductive surface. There are various known development systems for bringing toner particles to a latent image on a photoconductive surface. Single component development systems use a donor roll for transporting charged toner to the development nip defined by the donor roll and the photoconductive surface. In a scavengeless system the toner is developed on the latent image recorded on the photoconductive surface by the use of a donor roll with a plurality of electrode wires closely spaced therefrom in the development zone. An AC voltage is applied to the wires detaching the toner from the donor roll and forming a toner powder cloud in the development zone. The electrostatic fields generated by the latent image attract toner from the toner cloud to develop the latent image. In another type of scavengeless system called "hybrid scavengeless", a magnetic developer roll attracts developer from a reservoir. The developer includes carrier and toner. The toner is attracted from the carrier to a donor roll. The donor roll then carries the toner into proximity with the latent image.
The following disclosures may be relevant to various aspects of the present invention: U.S. Pat. No. 4,102,305. Patentee: Schwarz. Issued: Jul. 25, 1978. U.S. Pat. No. 4,565,438. Patentee: Folkins. Issued: Jan. 21, 1986. U.S. Pat. No. 4,868,600. Patentee: Hays et al. Issued: Sep. 19, 1989. U.S. patent application Ser. No. 07/563,026, (abandoned). Applicant: Floyd Jr. et al. Filed: Aug. 3, 1990. U.S. Pat. No. 5,206,693, Patentee Folkins, issued Apr. 27, 1993 .
The relevant portions of the foregoing disclosures may be briefly summarized as follows: U.S. Pat. No. 4,102,305 describes a developer roll that is electrically biased with an AC voltage superimposed over a DC voltage. U.S. Pat. No. 4,565,438 discloses an asymmetrical alternating electrical bias applied on a developer roller. U.S. Pat. No. 4,868,600 describes a scavengeless development system in which toner is detached from a donor roll and a powder cloud obtained by AC electrically biased electrode wires. The donor roll is electrically biased by a DC voltage. U.S. patent application Ser. No. 07/563,026 (abandoned) describes a magnetic roll for transporting developer material from a reservoir to a donor roll and electrode wires that are electrically biased to detach toner from the donor roll so as to form a toner cloud in the development zone. U.S. Pat. No. 5,206,693 describes a scavengeless development system in which toner is detached from a donor roll and a powder cloud obtained by Asymmetrical AC electrically biased electrode wires. The application of the asymmetrical AC in this patent is to the electrode wires and the purpose of the asymmetry is to reduce the wire vibration or "strobing" defect.
The present invention utilizes an asymmetric AC on the donor roll rather than the electrode wires in order to modify the position and trajectories of the toner particles in the powder cloud by modifying the fields in the space between the donor roll and the photoreceptor. The purpose of this asymmetry is to improve the non-interactive performance of the powder cloud against disturbing already developed images on the photoreceptor.